1. Field of the Invention
This invention relates to an electrical contact, and more particularly, to an electric contact employed, for example, in an electromagnetic relay.
2. Related Art
Sealed electromagnetic relays typically comprise a case including a cover and a terminal base which are made of a synthetic resin material. If the temperature of the environment where the relay is used is too high, organic gases are generated from the synthetic resin. Since the case is sealed, the gases remain inside the case and arcing and frictional energies released during the switching of the relay contacts cause the gases to oxidize to carbon. This carbon collects over the contacts to cause contact failure.
In the past, evacuation of the organic gases has been attempted in order to prevent carbon build up. It is impossible, however, to completely evacuate the organic gases, and the remaining gases consequently produce carbon. In electromagnetic relays for switching low-level signals, the electrical contacts are generally made of a precious metal such as gold. Since the precious metal serves as a catalyst, a great amount of carbon easily develops.
A life test of a contact which is made of 90% gold (Au) and 10% silver (Ag) by weight was conducted. The contact was tested with a load of 18 Vdc with 45 mAdc connected thereto and at a switching frequency of 2 Hz. The organic gases were previously evacuated before the test. FIG. 8 represents contact resistance versus the number of contact switchings as the result of the test. As FIG. 8 shows, the contact resistance increases to a great value, although it does not cause contact failure, as the number of switchings increases. In addition, FIG. 8 shows that the contact resistance excessively fluctuates.